A power unit which receives a commercial alternating current is incorporated into various electronic apparatuses such as AC adaptors, OA apparatuses, consumer apparatuses, etc. It is desired that these electronic apparatuses can suit commercial AC voltages of various countries in the world.
In order to suit electronic apparatuses to commercial AC voltages of various countries, some recent power units have an AC voltage detection circuit as an AC signal level detection circuit. The AC voltage detection circuit detects a power source voltage supplied from an AC power source and generates a determination signal for making the power source voltage identifiable. The power unit switches control manners and circuit protection manners based on the determination signal.
An example of conventional AC voltage detection circuit is shown in FIG. 10.
FIG. 10 is a circuit diagram showing a conventional AC voltage detection circuit.
This AC voltage detection circuit 10 is a circuit which generates a determination signal representing whether a power source voltage supplied from an AC power source 1 is, for example, 200 volts or 100 volts, and is provided between a positive electrode of a full-wave rectifying circuit 2 connected to the AC power source 1 and the ground.
The full-wave rectifying circuit 2 rectifies an AC voltage generated by the AC power source 1 and outputs a rectified voltage. A diode 11 whose anode is connected to he positive electrode of the full-wave rectifying circuit 2 charges a capacitor 12 connected between the cathode of this diode 11 and the ground when a voltage in a normal direction is applied thereto. The capacitor 12 is charged with the peak value of the rectified voltage generated by the full-wave rectifying circuit 2.
One terminal of a resistor 13 is connected to a connection node of the capacitor 12 and the cathode of the diode 11, and a resistor 14 is connected between the other terminal of the resistor 13 and the ground. These resistors 13 and 14 divide the charged voltage in the capacitor 12 and output voltages from the connection node between the resistor 13 and resistor 14.
The cathode of a zener diode 15 is connected is connected to the connection node between the resistor 13 and resistor 14, and the anode of the zener diode 15 is connected to the base of an NPN type transistor 16. The corrector of the transistor 16 is connected to the positive electrode of a DC power source 18 via a resistor 17, and the emitter of the transistor 16 is connected to the ground. The corrector of an NPN type transistor 20 is connected to the positive electrode of the DC power source 18 via a resistor 19. The corrector of the transistor 16 is connected to the base of the transistor 20, and the emitter of the transistor 20 is connected to the ground.
When the charged voltage in the capacitor 12 is raised and the voltage of the connection node between the resistor 13 and the resistor 14 is raised, the zener diode 15 reaches breakdown and flows a base current through the transistor 16. Due to this, the transistor 16 is switched on and lowers the corrector voltage of the transistor 16. When the corrector voltage of the transistor 16 is lowered, the transistor 20 is switched off. Due to the transistor 20 being switched off, the voltage of an output terminal connected to the corrector of the transistor 20 is raised. Due to this, that the AC power source 1 supplies a power source voltage of 200 volts is shown to the power unit, etc.
However, the conventional AC voltage detection circuit has the following problem.
Formation of a capacitor on a semiconductor substrate requires much larger area than formation of a transistor device and MOSFET. One capacitor that can actually and practically be formed on an IC (Integrated Circuit) is about 10 [pF].
However, the capacitor 12 shown in FIG. 10 is charged with the peak value of the commercial AC voltage. Since the cycle of the rectified voltage obtained by rectifying the AC voltage is long, about 10 [msec], the capacitor 12 needs to have capacitance of about 100 [nF] or more. Therefore, attempts to miniaturize the entire power unit by forming the AC voltage detection circuit on an IC and to delete the number of add-on components make it difficult to secure the area on the IC that allows capacitance of 100 [nF] or more to be obtained. Moreover, budgeting is unrealistic. Further, it is difficult for a capacitor on an IC chip to retain electric charges over a long cycle of a commercial frequency of several ten [msec], making it difficult to detect the peak voltage. That is, it becomes difficult to miniaturize the power unit by utilization of an IC and to lower the costs required, and reliability of the power unit might be damaged due to increase in the number of add-on components.